1. Field of the Invention
This invention relates to the generation of mechanical and/or electrical energy. More particularly, this invention relates to a combination process for the generation of electrical energy from a solid carbonaceous material such as coal, in which a base power load is generated by combusting a gas, and supplemental power for peak loads is generated by combustion of synthetic liquid hydrocarbons. The combustible gas for base-load power production and the liquid hydrocarbons used for peak-load power production are both produced in an integrated solid-fuel gasification and hydrocarbon synthesis process.
2. Description of the Prior Art
Solid carbonaceous materials have been used for a long time in the generation of power, particularly electrical energy. Generally coal is combusted with air and the exothermic heat of reaction is used to produce high-pressure steam, and then the steam in turn is expanded through a turbine to generate mechanical or electrical energy. Similarly, natural gas and other gaseous fuels have been combusted to form high-pressure steam for the generation of electrical power.
The electrical industry has developed highly efficient, large generators driven by expanding steam. However, one problem in the generation of electrical power from steam results from the greatly varying demand for electrical energy. Steam generators are not well suited for producing greatly varying amounts of steam, but rather are designed for base-load or constant-load types of operation. To provide for peak-load and reserve-load demands, the open-cycle gas turbines have generally been utilized because of their quick-startup capability and relatively low capital cost. Open-cycle turbines, however, generally require special fuels which are non-corrosive to the turbine blades. Generally it has been found to be uneconomical to combust coal or residual oils directly in the combustion chamber of a gas turbine, because the fuel contains high amounts of ash and sulfur. Due to the incomplete combustion, such high-ash solid fuels generally produce solid abrasive and corrosive particles. When such particles are entrained in the flue gas that is passed through the turbine, they deposit on the blades and erode the blade surfaces. When this corrosion occurs, the blade is damaged reducing the efficiency of the unit and the passages in the turbine become clogged. Further, the fine particles may deposit down-stream on heat-exchange surfaces and impair thermal efficiency. Similar problems are encountered when burning ash-producing liquid petroleum products. Such ash includes mineral compounds, as those found in crude oil. These compounds are concentrated in the residuals by the refining process and are supplemented by silica, iron, and sodium compounds which are picked up in shipment and in handling. Vanadium, nickel, sodium, sulfur and oxygen are the major components of the ash. After burning, they appear as metallic oxides, sulfates, vanadates and silicates. These compounds appear to erode the protective oxide films of high-temperature alloys used in gas turbines. Oxidation of the turbine blades is thereby accelerated, especially above about 1200.degree. F. Previous methods in which the fuel gas was cleaned prior to being introduced into the gas turbine were either impractical, unduly costly, or both.
The aforesaid problems of base-load and peak-load demand, combined with the special fuel requirements for gas turbines, are substantially avoided by the subject invention, which integrates the production of a base-load power generation combined with the production of a synthetic liquid fuel which is suitable for the generation of peak-load power in an open-cycle gas turbine.
Because coal and other solid carbonaceous materials often contain sulfur compounds, the combustion of coal for power production can also cause serious air pollution problems. Also, because of the greatly expanded volumes of gases produced after combustion, it is very expensive to remove the polluting sulfur compounds after combustion. These sulfur-removal problems and air-pollution problems have led to processes for the gasification of coal to produce a clean fuel gas wherein the sulfur is removed from the fuel prior to combustion. One problem, however, with such gasificaion processes is that only a low-BTU-value gas is produced, and it is generally not economical to transport a low-BTU-value gas over great distances. This has led to proposals for large on-site or "mine-mouth" gasification and power generation plants where the low-BTU-value gas is immediately converted to electrical power for transmission. Such on-site gasification and power generation processes solve the problem of low-BTU-gas transportation and sulfur-removal problems, but such gasification processes are not economical for producing greatly varying amounts of fuel as is needed for peak-load generation of power, either because it is too expensive to store gaseous products for subsequent use in gas turbines or because the capital expense of providing for greatly increasing the gas production rate and gas cleanup rate for peak-load demand is uneconomical.
The aforesaid problems are substantially avoided by the subject invention, which process provides for base-load and peak-load power demand. The process integrates the production of a combustible fuel gas for base-load power generation with a process for producing a liquid fuel which is free of sulfur and other impurities and which is suitable for storage and use for peak-load power generation using a gas turbine.
U.S. Pat. No. 3,868,817 discloses a process for the generation of mechanical and electrical power from a purified fuel gas produced from solid carbonaceous fuels. The purified fuel gas is used to generate power using gas turbines.
As described in the report entitled "New Fossil Fuel Power Plant Processes Based On Lurgi Pressure Gasification of Coal" by Paul F. H. Rudolph, delivered at ACS (American Chemical Society) on May 27, 1970, coal-burning gas turbines are used at Lunen in West Germany to drive electricity generators. As disclosed in that report, carbon or coal can be gasified in the presence of oxygen and H.sub.2 O. Gases from the gasification zone are purified to remove coal dust and fly ash, and also many other impurities such as vaporized ash, alkali and chlorine which are detrimental to the operation of gas turbines. After purification of the gases from the gasifications step, the gases are then combusted with air and then expanded through a gas turbine, which turbine is used to drive an electricity generator. The Rudolph report is directed to the gasification of coal and other similar materials with the subsequent purification and combustion of the gasification products. The Rudolph report does not disclose a gasification process followed by Fischer-Tropsch hydrocarbon synthesis and hydrogenation steps to produce liquid fuels particularly useful for generating power using gas turbines for peak-load demand.
Similar coal gasification and power generation processes are disclosed in U.S. Pats. Nos. 2,735,265 and 2,718,754.
The Fischer-Tropsch synthesis was used extensively in Germany during World War II to produce gasoline-boiling-range hydrocarbons. Today the Fischer-Tropsch synthesis is still being used commercially in South Africa, to produce straight-chain, high-boiling-range hydrocarbons with some medium-boiling oils, diesel oil, L-P gas, and oxygenated compounds. The existing commercial facilities using the Fischer-Tropsch process are detailed in a series of four articles by J. C. Hoogendoorn and J. M. Solomon, "Sasol: World's Largest Oil-From-Coal Plant", British Chemical Engineering, Part 1, May 1957, page 238; part 2, June 1957, page 308; Part 3, July 1957, page 368; Part 4, August 1957, page 418.